Your brain is the most complex piece of machinery ever created. It is a product of millions of years of evolution, with many of its deep-rooted structures still left unknown, and with many ideas such as consciousness and the emergent properties of the brain still left largely unknown in modern neuroscience. However, we do have a good understanding of the constituent parts of the brain, thanks to advancements in neuroanatomy!
Today, I’ll be giving you a tour around the brain, helping you to understand not only the general structure but also in getting you acquainted with the behemoth that is neuroanatomy, and even small tidbits here and there!
A Brief Introduction
Despite being only around three pounds, your brain is undoubtedly the most important organ in your body. It is what controls and regulates every single muscle interaction, governs hormone release, and is what gives you the ability to think, and, you know, be human. It’s no wonder why this small part of our body uses up 25 percent of all the calories we consume. This massive constraint would evolutionarily serve to limit our physical capabilities, but it gave us the power to create and wield the greatest weapon of all, technology.
Let’s first get something out of the way. Your brain is not made up of just neurons. In fact, only a quarter of your brain cells are actually neurons However, despite that, most of the other surrounding cells work to support the neuron anyways, so lets go meet them!
The superstars of the brain, the neurons. These cells have large treelike branches called dendrites(derived from “tree” in Greek) which reach out and receive an electrochemical signal from other neurons. Once they have received a signal, they can continue a cascade as the electrochemical signal travels down its axon and releases neurotransmitters through the axon terminals, which connect to the dendrites of other neurons.
These little molecular highways for electrical signals of the neurons are detrimental in allowing for fast transmission and interaction between largely spaced out neurons. Myelinated sheaths, composed mainly of fats, coat the outer sides of these axons, allowing electrochemical signals to travel much faster throughout the axon.
However, despite the neurons stealing the show, the glial cells are extremely important in the development and overall health and function of these neurons. The three glial cells, astrocytes, oligodendrocytes, and microglia, make up the bulk of the glial cells within the brain, and I’ll briefly go over each of their functions.
Astrocytes are a specific group of cells that act almost as the parents of the neurons. The multitude of astrocytes provides neurons with axon structure, synaptic support, and the supply of nutrients. Moreover, specific astrocytes are important in directing the flow of cerebrospinal fluid(CSF) and blood.
The myelinated sheaths that coat the axons and are crucial for the speed of electric signals are produced by the one and only, oligodendrocytes(in the central nervous system, Schwann cells do this in the peripheral nervous system). The importance of these myelinated sheaths cannot be understated, as there is an almost tenfold increase in myelinated vs unmyelinated/demyelinated speed. Multiple sclerosis(which leads to demyelination of the sheaths) leads to impaired cognitive functions, with symptoms such as fatigue, movement problems, and many other motor and neuronal functions come to be impaired.
And finally, we have the microglia cells, which are the macrophage equivalent in the brain. Since there is a blood-brain barrier(to prevent the spread of infection), normal immune cells are barred from entering the brain, and instead, these microglia cells act as the main defense of the brain, protecting the brain cells from pathogens and removing damaged neurons.
Now that we’ve gotten to know some more of the main players in the brain, it’s time to get a larger picture of what your brain actually looks like. The main things to focus on now will be the diagram shown above. Generally speaking, the outside of the brain is covered in grey matter(neurons) while the inside of the brain is made up of mostly white matter(axons), with some areas being made up of grey matter(hippocampus, thalamus, etc.). Another thing to keep in mind is that there are ventricles or openings in the brain that are filled with cerebrospinal fluid, but we’ll look more into that later.
Another important bit of terminology to remember is that the brain is wrinkled, with the little grooves in the brain being called sulci(singular: sulcus) and the hills/folds known as the gyri(singular: gyrus). This is important as we will be referring to specific gyri, on the brain that has specific functions. Although everyone’s brains are wrinkled differently, they still follow some structure, thus allowing for a function to be derived
Another thing you might be asking is, “why is the brain wrinkled at all? Well to explain it simply, the wrinkling allows for there to be a massive increase in surface area. This surface area means that there is more grey matter(neurons) and that helps to dramatically boost cognitive power and function. Moreover, those born without wrinkles(Lissencephaly) have massively impared cognitive function, and unfortunately don’t make it past childhood.
Now, before we take our little tour, we better get acquainted with the directions that are used to describe what part of the brain we are in. There are four general axes that you can find yourself in, and in a confusing fashion, each of the four axes has two names. But don’t worry, once you get this down, you’ll be able to know exactly where you are in the brain.
The axes are very simple, and they essentially outline the four cardinal directions, but just with fancy names. An important thing to point out is that the second way to describe the axes(dorsal, caudal, ventral, rostral) can also be used to orient yourself on the spinal cord or on an embryo and person. However, something to keep in mind, is that the orientation is slightly different in the spinal cord and embryo, but we can cover that later.
Superior/Dorsal— Upper part of the brain
Posterior/Caudal — Back part of the brain
Inferior/Ventral — The lower part of the brain
Anterior/Rostral — Front part of the brain
A way I like to remember the shifting axes as we move from the brain to the spinal cord is that the dorsal axis points outward, while the ventral axis always points inwards towards the internal organs. But to put it simply, just rotate the axes of the brain by 90 degrees and you will have the axes of the spinal cord, embryo, and in a person.
Often when engaging in diagrams and in gross specimens(dissections), the brain will be cut across a plane to give a better view of a region, or to display any irregularities or phenomena.
Coronal Plane — Cut from left to right side, allowing both hemispheres to be seen upright simultaneously
Sagittal Plane — Cut in line with the hemispheres(usually in the direct middle of them)
Horizontal Plane — Cut horizontally, from front to the back, usually exposing the whole cortex and limbic system.
With this knowledge under your belt, you are ready to join me as we ascend into the inner depths of the brain.
Parts of the Brain
And now we begin our journey! Above you can see the three major parts of the brain, those being the cerebrum, cerebellum, and brain stem. Although when we picture the brain, it is usually only the cerebrum that we think of, like glial cells, the cerebrum and brain stem are essential to allowing proper function within our brains!
I’ll dive into more detail later, but for now, it’s time to prepare ourselves for the lobes of the cerebrum. Starting from the front to back, we can see the frontal lobe, temporal lobe, parietal lobe, and occipital lobe.
The frontal lobe controls a lot of what we consider to be distinctly “human.” Processes such as executive functions(setting goals, planning, etc.) active thought, personality, and compelled motor control all are controlled through this lobe.
Next, we come upon the parietal lobe, which is situated just behind the frontal lobe and behind the central sulcus(little fissure running down both sides of the brain). It is responsible for intaking sensory information, such as touch and temperature, allowing us to feel and explore the world.
Although there is no definite area or line that marks the end of the parietal lobe, we still can’t forget the occipital lobe. Contrary to what you may think, this lobe is mainly responsible for sight. Yes, I know, it is literally as far away from the eyes that it can be, but this interesting fact will have to be a discussion for another day. All we need to know is that it is responsible for controlling sight, collecting and forming patterns out of what we see, and eventually giving us the power of vision!
Finally, we make our way to the temporal lobe, separated from the frontal and parietal lobe by the lateral fissure/sulcus. By extending an imaginary line backward from this fissure, we can break up the parietal lobe from the temporal lobe! What to remember about the temporal lobe, is that it gives you the final primary sense of hearing, but also encapsulates your memories and certain aspects of language processing.
I’ll get into more detail later, but this gives us the basic mapping of the brain(specifically the cortex)that we can build upon.
The “Skin” of the Brain
Your brain doesn’t just lie inside of your skull. While it “technically” does, it is actually surrounded by three protective layers known as the meninges. These layers help to protect the brain and also allow for the brain to swim/float in a nice pool of cerebrospinal fluid(CSF), which allows your soft and squishy brain(think soft tofu) to take minor impacts without deformation or damage.
The dura mater(meaning “tough mother” in Latin) is the most outermost meninge. As its name suggests, it’s a tough membrane, and it folds over the brain, allowing for blood to circulate and flow from the brain to the heart and vice versa. Since it closely covers the brain, it also forms folds within major gaps in the brain, such as the longitudinal cerebral fissure(fissure dividing the two hemispheres of the cortex). I’ll briefly go over the major folds that you will have to keep in mind.
Tentorium cerebelli(“tent of the cerebellum”) — Separates the occipital lobe from the cerebellum.
Falx Cerebri — This sickle-shaped fold helps to separate the two hemispheres of the cerebrum
Falx Cerebelli — Another sickle-shaped fold that separates the two hemispheres of the cerebellum
The final thing we need to remember before we leave the dura mater is that sinuses(openings that drain blood and CSF from the brain) are located on the outside of the dura mater. These veins drain into the jugular vein, eventually making their way to the heart, and then repeating the circulatory cycle all over again.
The leptomeninges are the colloquial term for the arachnoid and pia mater. These meninges line the brain much closely, conforming to the landscape of the wrinkled brain.
The space between these meninges(called the subarachnoid space)is where the CSF is able to reside. Although there is only 150 mL, roughly a third of a plastic water bottle, this small amount cushions and floats the brain, making it almost 60 times lighter.
Before we begin talking too deeply about the structures in the brain, I would like to complete our exploration of CSF by looking at the ventricular system which is comprised of small openings in the brain that help to produce and circulate CSF. There are four main landmarks to remember, and we’ll start with the lateral ventricles first.
The lateral ventricles are the largest ventricles, swinging up and around the cortex. To describe areas of the lateral ventricle, we use the word “horn” to denote any protrusions, big or small, that are in any region of the brain. And from that, we can denote the anterior, posterior, or inferior horns of the lateral ventricle. It is important to remember that each hemisphere has one lateral ventricle, thus making them the 1st and 2nd ventricles respectively.
And in the anterior horn of the lateral ventricles, we meet the intraventricular foramen(“opening” in Latin) that connects us to the 3rd ventricle. It is very flat in shape and is located between the two thalami(we’ll talk about them in the ventricular system). Since the thalami are so close together, they create a gap in the third ventricle, which you can see in the diagram.
As we travel down through the third ventricle, we eventually make our way into the aqueduct, a small tube that connects us to the fourth ventricle. It is sandwiched in between the brain stem and the cerebellum, which leads to this pyramid shape that it takes.
The flow of cerebrospinal fluid follows this path that we went on, with the current originating from the lateral ventricles, going into the third ventricle, aqueduct, and then the fourth ventricle. Once it reaches the fourth ventricle, it flows out in small openings called the cistern manga, and into the subarachnoid space in the leptomeninges, and helping to cushion the brain from injuries.
This cerebrospinal fluid that I was just talking about plays an integral role in protecting the actual brain and spinal cord.
But what is it?
It is a clear fluid, made almost entirely of plasma, with very few cells and small proteins being present. This fluid is distinct from the blood that circulates the whole body, leading to a blood/brain barrier, where the brain is sectioned off from the other parts of the body, only allowing small molecules passage into the CSF and protecting the brain from peripheral damage, such as an immune response or an infection.
With all this talk about CSF, we haven’t even mentioned where this CSF comes from! Look no further than the choroid plexus, regions of CSF producing cells. They are located in all of the ventricles and help to replenish CSF as it eventually drains into the dural sinuses.
The final housekeeping item that we need to go over is the blood supply to the brain. It is crucial that we discuss this, especially since the brain uses up approximately 25 percent of all of the nutrients of the body. The flow to the brain is actually really simple, and we will focus on the main arteries that lead into the brain.
The common carotid arteries which originate from the body split into two, becoming the internal and external carotid arteries. The external carotid arteries help to line the outer cortex of the brain, with the inner carotid arteries providing blood to the inner parts of the brain. Not so difficult, huh?
Next, we have the vertebral arteries, that line both sides of the spinal cord and eventually come together to form the basilar artery, which is in the underside of the Circle of Willis. The Circle of Willis itself allows for a more distributed flow of blood, providing blood to the pituitary stalk(pituitary gland releases a significant amount of the hormones in your body) and also allowing for communication of the blood flow between the forebrain, and the hindbrain(brain stem).
We’ve already gone over the major lobes of the cortex, but now we’ll dive a bit more deeply into the major areas within each lobe. As you can see from the diagram above, there are complex subdivisions of the areas of the lobe. It is important to remember that the part of the body that the brain controls is swapped. For example, if you wanted to move your right hand, the commands to do so would come from your left hemisphere, and vice versa. This interesting occurrence is called decussation, and I’ll go more deeply into it later.
The motor strip is the gyrus of the cortex that is directly responsible for the voluntary movement of different areas of your body. It is located in front of the central sulcus, which divides the front and back hemispheres of the brain, and as you can see from the diagram up top, most of your motor neurons are devoted to the face and hands, with little focus on other areas, such as the legs, arms, or chest.
Another way to better visualize this relationship is freaky but also extremely interesting being called the “homunculus.” The way it works is that the size of different parts of the body correlates to the number of neurons that control them, creating this beast that sorta reminds me of a horse, just with extremely large hands.
This is an important reminder of just how detrimental hands are in our evolution. This fine motor control with our fingers allowed our ancestors to craft tools and build fine structures. Even now, we are pushing the limits of our hands, through our dedication to instruments or through art. Our dependence on our hands allowed us to create, to build, to invent. And it is this that is what predominantly allowed us to become one of the most dominant species once we were able to form communities together.
The somatosensory cortex is devoted to sensing sensations such as pressure, touch, pain, and temperature and is located on the gyrus that is adjacent to the motor cortex, on the other side of the central sulcus. It is absolutely crucial in helping us gather information, and follows a similar proportion of neurons that the motor cortex does, with some minor adjustments.
Broca’s and Wernick’s Area
These two final areas that we will explore in the cortex are both responsible for the processing and synthesis of speech. As we can remember from the function of the different lobes, Broca’s area(located in the frontal lobe) is responsible for the production of speech, while Wernicke’s area is located in the temporal lobe, making it responsible for the audible comprehension of speech.
Both of these areas are located on the left side of the brain, which is responsible for the
Damage to these areas leads to aphasia, the inability of proper communication. Broca’s aphasia and Wernicke’s aphasia are distinctly different, and the two videos down below highlight the importance that each of these areas has to your communication.
The corpus callosum is one of the most recognizable parts of the brain. Its bundle of fibral axons that bridges the left and right hemispheres and is directly above both the lateral ventricles. This connectivity allows for the quick transfer of neuronal signals between both hemispheres!
As we descend into the deeper and more primordial parts of the brain, we reach the limbic system. This region is heavily studied in fields such as psychology, as this is the area of the brain that regulates the release of hormones, and as such controls your emotions. Although often separately studied, it is detrimental to understand that many parts of the brain interact together, and as such, this limbic system that regulates your emotions has major effects on other regions such as the “rational” prefrontal cortex.
However, what's really peculiar is that these regions of the brain are plastic, being able to change their sizes. For example, your hippocampus and amygdala will grow and shrink as stressful situations arise. And this interesting fact might make you more conscious of not only the mental repercussions but the damning physical consequences of being constantly stressed.
Being one of the main regions of the limbic system, the hippocampus is directly responsible for the storage of long-term memory. Within the limbic system, many other regions are competing to influence the hippocampus, like our next runner up, the amygdala.
An interesting little tidbit is that the hippocampus is actually named after a seahorse, as it resembles it in shape! Fittingly, the scientific genus of the seahorse is the hippocampus, a horse sea monster.
This region is most associated with regulating fear response and stress. And from that, it can be clear why the amygdala and hippocampus almost go hand and hand, with horrible and fearful memories often searing their way into the minds of people and having lasting effects.
The thalamus regulates very basically motor control and sensory signals between the brain cortex and the body. It is also associated with being alert and general consciousness(just the awareness part of consciousness though).
Below the thalami are the two hypothalami. These guys are vastly important in the endocrine system(hormonal system), since they are located right above the pituitary gland, which releases hormones to every part of your body. This important link allows for the limbic system to regulate emotion and emotional responses as well.
Although previously thought of as the region that regulates aggression, the hypothalamus actually regulates hunger, and the survival instincts that we all possess.
The nucleus accumbens is a nucleus(cluster of neurons/grey matter) that facilitates the release of dopamine and is keenly affected by all types of drugs such as cocaine. Although this region is known for its reward system, the reward itself does not exactly produce the release of dopamines.
In the beginning, it is the case that dopamine is released when a reward is completed, but as things progress, the dopamines release when an action that precedes the actual reward occurs. For example, if we were to ring a bell, and if a dog were to sit down, it would get food, eventually, just the ringing of the bell would release the dopamine since the dog already knows the jig.
Connection with the Prefrontal Cortex
However, an interesting breakthrough that was made between the limbic system was its connection with the actual cortex. Although previously thought of as a separate system, this linkage with the prefrontal cortex, which is associated with the executive decision and what makes humans distinct from other primates and animals, means that we are not perfectly rational, and Descarte’s separation of mind and body was actually just dead wrong.
Inherently this makes intuitive sense, as if we were to be hungry, it would be optimal to address this hunger before we were to, you know, die. However, another interesting parallel can be drawn between the actual connection between the mind and the body. Some anti-depressants work by actually relaxing the muscles in your body. How can that be? Well, as I stated before, this connection between the prefrontal cortex, and thus some sensory information, is directly affected by the limbic system. Vsauce did a great video demonstrating just how much the physical state affects the mental state.
And now that we have finished visiting the cerebrum, we happen to come upon the “hindbrain” or better known as the cerebellum. Although it is small, it is absolutely crucial in marrying voluntary movement instructions from the cortex, and sensory information from the body. As shown in the diagram above, there are three main regions that we like to divide the cerebellum into.
The anterior lobe, also known as the cerebrocerebellum is the most important region in coordinating voluntary movement and guiding it into the body. Keep in mind that it is also influenced partly by the visual information that it takes in as well.
Below that we find the posterior lobe or spinocerebellum. This area acts as the error correction system for voluntary movement. Think of it as a constantly updating system that corrects for movement of the body.
And finally, we reach the floccundular lobe or the vestibulocerebellum. Although minuscule compared to the other lobes, it is important in ocular reflexes and also maintaining balance, as it works in part with the vestibular system of the ear(controls balance).
The brainstem is the final region of the brain we’ll tackle for today, and it is the most primordial/old part of the brain. Thus it contains basic autonomic functions such as breathing, keeping the heart beating, and blinking. Moreover, it directly connects the brain to the spinal cord. Although it might seem a bit boring, sometimes we don’t appreciate how important this is until we don’t have it.
Ondine’s curse is a disease where you cannot automatically breathe, and must be conscious to be able to properly breathe. This absolute nightmare leads to deaths of asphyxiation(no breathing while sleeping), or from exhaustion(from not being able to sleep).
Another important factor from this is reflexes, and we have specific nerve tracts(12 to be exact)that lead directly from the face regions to the different areas of the brain stem, but I won’t go into too much detail about those, as so not to increase the amount of information that I am sort of firehosing you with right now.
Well, with that all laid out, let’s take a look at the midbrain. This part of the brain lies in the middle(go figure) of the brain. It makes up the top of what we know as the brain stem and is crucial in serving as a pathway between the body and the brain, as I mentioned previously. Keep in mind that this diagram is cut across the horizontal plane, so the top of the diagram is on pointing to the posterior side of the brain, and the bottom pointing inward, anteriorly.
Let's take a better look at this cross-section of the midbrain. This is a cut through the horizontal plane of the midbrain, and the most immediate things that help me identify the midbrain is the opening of the aqueduct, and the substantial nigra.
I’ll give a brief overview of the major nuclei and regions to the midbrain. First and foremost, the substantia nigra. This region contains more dopaminergic neurons(neurons that transmit dopamine) and has neuromelanin(skin pigment) that gives this region this dark hue. Being that the neurons here regulate dopamine, it is crucial in managing reward responses and movement.
The spinothalamic tract and dorsal column medial lemniscus (DCML) are part of a larger pathway of axons(which we will get to in a bit). All we need to know right now is that this area is part of a long chain that extends down the spinal cord into the cerebrum, responsible for gathering sensations from the body and moving them into the cortex for analysis and response.
The crus cerebri are the anterior side of the cerebral peduncles(shown above) which help to connect the midbrain and brain stem to the cerebrum. Not only that, but the cerebral peduncles contain major motor tracts that extend from the cortex to the spinal cord.
As we lower ourselves down the brain stem, we reach the pons(“bridge” in Latin), which acts as a bridge between the midbrain and the medulla. Cross-sections of the pons contain large striated patterns, which are the large fibers that you can see above. The fourth ventricle is located right behind the pons, situated between the pons and the cerebellum.
Although there are many important nuclei, in the pons, it seems that it is time for us to descend even further, into the medulla.
We’ve now reached the final part of the brain stem, the medulla. The medulla sits right above the spinal cord, and is responsible for not only carrying signals from the spinal cord to the higher reaches of the brain, but is also responsible for the decussation of pathways.
This decussation occurs specifically in the pyramids of the medulla, and what happens is the crossing of different pathways, with the end behavior being that the right hemisphere of the brain controls the left half of the body and vice versa.
Nerves of the Brain
Before we finish our look into the brain, it would be crucial to outline the twelve different cranial nerves that lead from the head directly to the brain. There are very small openings within the skull that allow for each pair of nerves to cross between the head to the brain. First lets take a look at the two nerves that lead directly into the cerebrum.
The first cranial nerve is the olfactory bulb, which lies on the inferior side of the frontal lobe. As its name implies, it is responsible for smell, but also crucial in taste, as smell plays a major role in taste as well.
The optic nerves form what is called the optic chiasm, as they meet and decussate. However, the decussation is different than the decussation within the pyramids of the medulla. It is instead decussation of the right and left visual field, with the right hemisphere being responsible for the right half of retinal signals(left visual field)and with the left hemisphere responsible for processing the left half of the visual field(right visual field)as shown above.
This is sort of a tangent, but it really helps to illustrate what I meant by the opposite nature of the retina and the visual fields. What we see here is the flipping of images as they pass through our eyes and hit the back of our retina. Since the retina lines the back of the eyeball, all visual stimuli must pass through this lens the light from the bottom visual field hits the top of the retina, and vice versa with the left, right, and upper visual fields. However, our brain has adapted to this and manages to flip everything, and so we are accustomed to this seemingly unintuitive way to see.
Oculomotor, Trochlear, and Abducens (3, 4, and 6)
Now, we arrive upon the two cranial nerves of the midbrain(oculomotor and trochlear) and the abducens, which are located in the pons. These nerves are directly responsible for controlling specific muscles that dictate the movement of the eyes.
Shown above, we can see that the third cranial nerve controls most of the eye movement, looking up, down, and outwards. However, the abducens are responsible for inward movement of the eyes, and the trochlear nerve responsible for the superior oblique, which helps to intort and extort(rotate) the eyes for focus.
Damage to areas of the brain stem that carry these signals leads to nystagmus, where the eyes are unable to properly pursue a target, slightly jerk when looking laterally or horizontally, or don’t adduct(line up the pupils with the target).
The trigeminal nerve is an intricate nerve that collects sensations from all over the face and leads to the pons. It is often considered the most complex nerve due to its numerous branches that include the three regions shown above in the diagram.
Another nerve that originates from the pons is the facial nerve, which responsible for facial expression and sensations of taste on the frontal two-thirds of the tongue, with the posterior one third being the responsibility of the ninth cranial nerve.
The final nerve of the pons is the vestibulocochlear nerve, which receives information from the vestibule and cochlea of the inner ear. These two areas of the ear are responsible for maintaining balance and auditory information, thus being crucial in coordinating motor movements with other parts of the body. With damage to this nerve or the inner ear, something as simple as walking can prove to be a challenge.
The ninth cranial nerve(glossopharyngeal is just a mouthful) specifically innervates the lower facial muscles(mostly the pharynx/throat) and is responsible for responses such as the gag reflex or swallowing. And, as I stated previously, it is responsible for the sensations of the posterior one-third of the tongue.
As you can clearly tell from the diagram above, the vagus nerve plays an absolutely integral role in the communication of the internal organs with the brain. The nerve is responsible for the parasympathetic(automatic response when at rest) regulation of the internal organs, and is responsible for controlling areas such as the digestive system, the heart, and the lungs.
Accessory and Hypoglossal Nerve (11 and 12)
And we’ve sadly reached the end of our visit with the cranial nerves. The accessory nerve and hypoglossal nerve both originate within the medulla, with the accessory nerve controlling muscles near the upper shoulder/neck region, and with the hypoglossal nerve controlling the intrinsic(the actual tongue)and extrinsic muscles(not the tongue)of the tongue.
Although not a part of the brain, the spinal cord is what facilitates the communication of the brain with the rest of the body. In the brain, we saw that on the outside there was a layer of grey matter, that covered the large axon highways of the inner brain(white matter). However, in the spinal cord, this is exactly backward, with the grey matter being on the inside, and the white matter on the outside.
What's also interesting is that the amount of grey matter there is shifts with each region of the body, with the areas of the spinal cord governing the hands having more grey matter than say the abdomen or legs.
This grey matter in the spinal cord are mainly for reflexes, as sometimes it would be too late if we were to have the sensory signal make its way up to the brain and then back down again. Simple reflexes such as a painful stimulus will instantly activate these neurons, and thus allow for quick reactions.
All of these signals reach the spinal cord through the ganglion, which extend out to the body and gathering information from the peripheral nervous system(PNS). These ganglia surround the vertebrae of the spine, and any minor damage to the spinal cord or ganglion can lead to loss of sensory input and even paralysis.
The spinal cord does not extend all the way down, to the pelvis, and in reality, it is the cauda equina that is responsible for acquiring the peripheral information of the lower half of the body.
They almost mimic the function of the ganglion, having large root-like structures. This cauda equina ends near the tailbone, with the sciatic nerve going down to the lower legs to connect to the PNS.
As we complete our tour, I would like to discuss the role that evolution had to shaping this complexity that I just walked you through in the brain. Mother nature is not an inventor, she is a tinkerer. She tries to make the best out of what is already there. And from that, we can see some interesting phenomena within the brain, especially with decussation, or even with synaptic connections.
In order to move your pointer finger, it isn’t just a direct instruction to move just one finger. In reality, it is a layer of two mechanisms. The first synapse moves all fingers of the hand, in a grasping motion. The second synapse is an inhibitory synapse, stopping the motion of the other fingers and only allowing for the motion of the pointer fingers.
This example demonstrates that it is often that many biological systems are not perfectly designed, with many systems overlapping to cancel the previous archaic and older pathways. Although a complete reconstruction and rewiring of the systems within our body would drastically improve our capabilities, evolution cares little for perfection.
In biology we almost never see perfection. Instead, we see adaptation, change, and constant readjustments. Being that we ourselves are biological systems, this rule applies to us as well. In our ever-changing world, this definition of perfection will never stay in one solid place, and it is up to us to constantly adapt and update our maps of reality to truly improve and grow.
“It is not the strongest or most intelligent who will survive, but those who can best manage change. ” — Charles Darwin.